1. Field of the Invention
This invention relates to the induction of stock activity and the control of drainage in a Fourdrinier table, particularly by the use of lifting variable inertial stimulation blades which can further include limited-vent indented surfaces.
2. Description of the Prior Art
Stock activity in the early part of a Fourdrinier table is critical to the production of a good sheet of paper. Generally, stock activity can be defined as turbulence in the fiber-water slurry on the forming fabric. This turbulence takes place in all three dimensions. Activity plays a major part in developing good formation by impeding stratification of the sheet as it is formed, by breaking up fiber flocs, and by causing fiber orientation to be random. Typically, stock activity quality is inversely proportional to water removal from the sheet. That is, activity is typically enhanced if dewatering is retarded. As water is removed, activity becomes more difficult because the sheet becomes set, and because water, which is the primary media in which the activity takes place, becomes scarcer. Good paper machine operation is therefore a balance between activity and drainage.
There are a number of conventional methods to promote activity and drainage. A table roll causes a large positive pressure pulse to be applied to the sheet resulting from water under the forming fabric being forced into the incoming nip formed by the roll and forming fabric. This positive pulse has a positive effect on stock activity by causing flow perpendicular to the sheet surface. Similarly, on the exiting side of the roll, large negative pressures are generated, which greatly enhance drainage. Table rolls are generally limited to relatively slow machines because at high speeds, the positive and negative pulse amplitudes become excessively large. Foils are used to promote and control activity and drainage. A vacuum pulse is generated by the nip formed by the forming fabric and conventional foil as the fabric passes over the foil. Activity is generated by using a number of consecutively placed foils, encouraging a positively reinforced activity in the stock. Another type of foil, sometimes referred to as a "posi-blade", incorporates a positive incoming nip to generate a positive and negative pressure pulse. The amplitude of the pressure pulse is determined in a large part by the angle formed by the fabric and the incoming edge of the foil. This type of foil simulates a table roll, but with much lower amplitude positive and negative pressure pulses. The amplitudes are determined by the speed of the machine and the angles of the foils.
Often, Fourdrinier tables are mechanically shaken to promote stock activity, especially on slower, narrower machines. While the shaking might be a good way to enhance formation it is undesirable because it is difficult and expensive to control and maintain, and generally punishing on the equipment on and around the Fourdrinier Table. For paper making in general, most activity inducing systems have the negative feature of excessive drainage.
In patent application Ser. No. 08/600,833, entitled "Velocity Induced Drainage Method and Unit", filed on Feb. 12, 1996, U.S. Pat. No. 5,830,322, discloses an alternate way of creating activity and drainage. The apparatus disclosed therein, and illustrated herein as FIG. 1, decouples activity and drainage and therefore provides independent control and optimization of activity and drainage. The device typically uses a long blade with a controlled, at least partially non-flat or undulated, surface to induce initial activity in the sheet, and limits the flow downstream of the blade through placement of a trail blade to control drainage. Drainage is enhanced if the area between the long blade, the forming fabric and trail blade remains flooded and surface tension is maintained between the water above and below the fabric. However, the implementation of this device has revealed phenomena previously not fully appreciated. The first occurs in the "counterflow zone" over the long blade, particularly at the undulated portion, where the incompressible fluid is pumped through the forming fabric. This was expected. However, the second activity is much more vigorous and had not been fully appreciated. As the forming fabric spans the relatively long distance between the lead edge of the long blade and the trail blade, it deflects downwardly because of the forces acting on it. These forces are gravitational and also result from the vacuum induction as the fabric travels along the long blade. The latter predominates by far. The wire takes on the shape of a skewed catenary as the forces are asymmetrical along the wire between the support points. If the long blade is high enough or the fabric deflection is severe enough, the wire will contact the long blade and the catenary shape will be further distorted. The activity is induced when the fabric reaches the trail blade. The fabric path must make a rapid transition from the deflected state to the horizontal state very quickly at the leading edge of the trail blade because of the high tensions acting on the fabric path. The fabric path therefore changes sharply as the fabric travels around the sharp leading edge of the trail blade. Inertial forces prevent the fluid slurry of the paper sheet from following the fabric, and inertial activity is induced as the sheet lifts vertically.
Submerged drainage in a Fourdrinier fabric is disclosed by U.S. Pat. No. 5,522,969 to Corbellini et al. entitled "Submerged Drainage Method for Forming and Dewatering a Web on a Fourdrinier Fabric" and U.S. Pat. No. 5,242,547 to Corbellini et al. entitled "Submerged Drainage System for Forming and Dewatering a Web on a Fourdrinier Fabric". Positional control of elements in papermaking apparatus is disclosed in U.S. Pat. No. 5,486,270 to Schiel entitled "Angularly Adjustable Drainage Foil for Paper Machines"; U.S. Pat. No. 5,421,961 to Miller entitled "Forming Board Position Control System"; U.S. Pat. No. 5,262,010 to Bubik et al. entitled "Dewatering Device with Adjustable Force Elements for the Web-Forming Section of a Papermaking Machine"; and U.S. Pat. No. 5,221,438 to Takeuchi et al. entitled "Supporting Device for Dewatering Elements".
U.S. Pat. No. 3,595,747 to Walser entitled "Suction Box Covers with Rows of Drainage Openings for Uniform Dewatering" and U.S. Pat. No. 5,562,807 to Baluha entitled "Cross Direction Fiber Movement and Dewatering Device".